1. Field of Invention
This invention relates generally to motors, and more particularly, to hydraulic motors that produce work when a working fluid is pumped through it.
2. Description of Related Art
Today's downhole drilling motors usually are of the convoluted helical gear expansible chamber construction because of their high power performance and relatively thin profile and because the drilling fluid is pumped through the motor to operate the motor and is used to wash the chips away from the drilling area. These motors are capable of providing direct drive for the drill bit and can be used in directional drilling or deep drilling. In the typical design the working portion of the motor comprises an outer housing having an internal multi-lobed stator mounted therein and a multi-lobed rotor disposed within the stator. Generally, the rotor has one less lobe than the stator to facilitate pumping rotation. The rotor and stator both have helical lobes and their lobes engage to form sealing surfaces which are acted on by the drilling fluid to drive the rotor within the stator. In the case of a helical gear pump, the rotor is turned by an external power source to facilitate pumping of the fluid. In other words, a downhole drilling motor uses pumped fluid to rotate the rotor while the helical gear pump turns the rotor to pump fluid. In prior systems, one or the other of the rotor/stator shape is made of an elastomeric material to maintain a seal there between, as well as to allow the complex shape to be manufactured.
One of the primary problems encountered when using the standard style of stators is that the profile lobes are typically formed entirely of elastomer. See FIG. 1. Since swelling due to thermal expansion or chemical absorption is proportional to the elastomer thickness different parts of the profile expand differently. Moineau, U.S. Pat. No. 1,892,217 and Bourke, U.S. Pat. No. 3,771,906 disclose stators constructed from elastomeric materials of varying section thickness of the elastomer. U.S. Pat. No. 5,832,604 to Johnson et al. discloses a rigid stator made of a disk stack and elastomeric lining. The elastomer allows the stator to begin a run with a tight seal around a rotor from the elastomer or rubber lining, which gives the motor high efficiency.
FIGS. 1-2 depict a prior art metal rotor and a rubber-lined stator having a rubber or elastomeric stator lining and a metal stator tube. This conventional power section stator configuration comprises a profiled rubber section where the rubber has varying thickness. FIGS. 3-4 depict a prior art metal rotor and a metal on metal stator having a metal stator lining and a metal stator tube. Furthermore, FIGS. 5-6 depict another prior art version of a rubber-lined metal stator that utilizes an internally-shaped tube and a profiled rubber section. In this version, referred to as an “even wall power section,” the profile of the rubber has an even thickness, as shown most clearly in FIG. 6, as opposed to uneven thickness of the rubber portion in FIG. 2.
However, under difficult conditions of load and temperature, the rubber may not last long enough to finish the planned run. The usual failure mechanism for these conventional power section stators and even wall power section stators is chunking of the rubber as it fatigues due to cyclic loading. The chunking usually commences at the end of the stator where the rotor is connected to the bearing assembly of the motor due to the sideload from the constant velocity joint or flex shaft. Correcting this failure would normally require a time consuming and costly trip out of the well to change the stator. The inventors have contemplated and solved this problem with the realization that a motor with an elastomeric stator could keep operating under such conditions with some modifications that would increase durability and reliability in operation, as will be discussed in greater detail below.
All references cited herein are incorporated herein by reference in their entireties.